This invention relates to improving the cooling of armature bars at the generator endwinding region, and specifically, to a new generator gas shield that more effectively distributes cooling flow to that region.
In the process of producing electricity, power generators also create heat that must be dissipated away from the generator. Heat occurs in generators due primarily to friction and current. Frictional heating occurs as the rotor spins at high speed in the generator. Similarly, heating also occurs as current flows through the rotor and stator coils, as these coils rotate relative to one another in the magnetic fields of the generator. Generators are typically equipped with cooling systems to transfer heat from the stator and rotor away from the generator.
The stator winding coils are made up of bundles of strands of insulated copper wires that are embedded in the slots of the stator core. At the ends of each coil, the copper wires are brazed together to form the armature bar leads. The armature bars are separated by armature bar blocks and are bonded with binding bands, with the loop ends enclosed in series loop caps. Consequently, cooling flows passing through the armature bars are considerably complex.
In modern high-powered, large-size generators, gas shields are used for two purposes: (1) to separate the low pressure side of the rotor fan from the high pressure side, and (2) to direct cooling flows toward the armature bars and current connection rings. However, the cooling purpose has been generally overlooked by generator design engineers.
Presently there are two general configurations of gas shields: contoured and conical. Contoured gas shields are used with axial fans to provide airflow into the inlet side of the fan. Conical (or straight) gas shields are used with axial flow fans where flow entry into the fan is not critical. Gas shields are generally mounted to the frame section plate just inboard of the end plate. When they are mounted to an intermediate inner end shield, they are called nozzle shields. This invention is related to the type of contoured gas shields that are used in both forward-flow and reverse-flow generators.
In a typical contoured gas shield arrangement, the gas shield is mounted on the frame section plate. A radial clearance of 0.060xe2x80x3xc2x10.010xe2x80x3 is held between the fan blade tips and a toothed insert in the gas shield. However, this design cannot guarantee sufficient cooling for armature bars, and especially for series loop caps. As a result, hot spots may appear on the armature bars.
In order to improve cooling effectiveness of armature bars, a robust design for gas shields is highly desirable. We have found that several design changes in known generator gas shield configurations can be implemented for better controlling the flow distribution at the fan flow exit, including: (a) Optimizing the gas shield profile A significant change in the gas shield redesign is to use an aerodynamically smooth surface at the fan flow exit. Specifically, the gas shield includes an annular ring body with an outer, radially extending wall terminating at a first free end on the fan inlet side. The annular ring body also projects beyond the fan flow exit via an axially extending portion that surrounds a center opening, with the axially extending portion bent back towards the armature bar. Thus, on the exit side of the fan, the shield leads the cooling gas to flow along its aerodynamically smooth surface to cool the armature bars. A gap formed between the second free end of the gas shield and the armature bar edge can be used to control the flow rates through the main body of the armature bar and towards the series loop caps.
(b) Adding ventilation holes In order to bring the cooling gas directly to the series loop caps, a number of ventilation holes are provided circumferentially near the gas shield edge on the fan flow exit or outlet side. These ventilation holes serve as gas nozzles to impinge the cooling gas directly on the series loop cap surfaces (as well as current connection rings). The heat transfer coefficient associated with directly impinging jets is several times higher than normal through-flow coefficients, so that the armature bar realizes much higher heat transfer.
With the above changes, the gas shield in accordance with this invention provides better control of the cooling flow, in that the latter is split at the front of the centering ring to enter into the rotor-subslot, rotor-stator gap, and armature bars. The series loop caps are cooled by flow from ventilation holes and through the gap between the gas shield and armature bars.
Accordingly, in its broader aspects, the invention relates to a generator gas shield comprising an annular ring body having an outer radially extending flange terminating at a first free end of a first diameter; a curved inlet portion; a substantially axial portion surrounding a center opening, and a curved outlet portion terminating at a second free end of a second diameter smaller than the first diameter.
In another aspect, the invention relates to generator comprising a rotor and a stator, and an axial flow fan, the stator having an endwinding region including circumferentially spaced armature bars arranged about the rotor, with a radial cooling gap between the stator and rotor; the armature bars terminating at loops enclosed within a corresponding number of series loop caps; and an annular gas shield having an outer radially extending flange secured to a section plate of the stator; a curved inlet portion; an axial portion defining a center opening surrounding the axial flow fan and including a seal insert adapted to cooperate with and establish a seal with blades of the axial flow fan; and a curved outlet portion terminating at a location proximate the armature bars.
In still another aspect, the invention relates to a method of cooling armature bars in a generator comprising a rotor and a stator, and an axial flow fan, the stator having an endwinding region including circumferentially spaced armature bars arranged about the rotor, with a radial cooling gap between the stator and rotor; the armature bars terminating at loops enclosed within a corresponding number of series loop caps; the method comprising a) providing an axial flow fan radially inward of the series loop caps to direct cooling air axially into the endwinding region; b) directing cooling air from the axial flow fan along a smooth surface to the armature bars; and c) providing nozzles in the smooth surface adjacent the series loop caps to impingement cool the series loop caps with the cooling air.